1. Field of the Invention
The present invention relates to an automatic transmission, and in particular, to a frictional engagement device for engaging and disengaging a rotary element of a planetary gear set to/from a stationary member.
2. Description of the Background Art
In some automatic transmissions, a rotary element (a sun gear, a carrier or a ring gear) of a planetary gear set is disposed to engage a stationary component, such as a casing, through two paths, e.g. a brake and a one-way clutch. In the prior art, the above disposition is achieved by structures, as disclosed in the U.S. Pat. Specification No. 3,831,464, in which the inner race of the one-way clutch is bolted to a brake cylinder, and the outer race is splined through a sleeve to the ring gear of the planetary gear set.
If the above arrangement is applied to a brake part for a reverse driving, this part must bear a large torque, so that the force for engaging or holding the brake must be increased. However, if the number of friction members constituting the brake is increased and/or the hydraulic pressure for operating the brake is increased for the above purpose, the increase in the number of friction members of the frictionally engaging element may result in an increase in the axial length of the transmission and torque loss due to drag in the disengaged state, and the increased hydraulic pressure may require substitution of stronger, i.e. heavier and larger, parts. Therefore, it is preferable to increase the pressure receiving area of an operating piston in order to increase the torque capacity of the brake without causing the above disadvantage.
In recent years, there is a tendency with automatic transmissions toward increase in the number of speed change ranges, reduction in size and lighter weight construction with a view toward enhancement of driving performance of the vehicle, comfort of ride, reduction of fuel consumption and other advantages. In order to satisfy two conflicting demands, i.e. increase in the number of speed change ranges, which increases the number of components of the automatic transmission, and reduction of the size and weight, improvements are required for minimizing the sizes of members in each part of the automatic transmission and for more closely arranging the members.
In the prior art, the inner race of is merely bolted to the brake cylinder, so that the inner race and the cylinder wall must have sufficient thicknesses for insertion of the bolts, which requires increase in radial size and conversely reduces the pressure receiving area of the piston.
Japanese Laid-Open Patent Publication No. 61-45148 discloses a construction for increasing the pressure receiving area of the piston. This construction employs two axially aligned brake pistons, which together form a force magnifier. Although this construction can avoid an increase in the radial size, the axial size disadvantageously increases.
Accordingly, it is an object of the invention to provide a rotary member engaging device, in which a rotary element of a planetary gear set is coupled with a stationary member through two paths formed of a frictionally engaging element and a one-way clutch, and more particularly, to provide a rotary member engaging device in which the pressure receiving area of a piston for operating the brake is sufficiently large and thus has sufficient torque capacity, without an increase in size.